The present invention relates generally to methods and apparatus for converting an absolute measurement signal to an incremental signal. In particular, one embodiment of the invention relates to a method and apparatus for converting a sensor measurement to an xe2x80x9cA quad Bxe2x80x9d incremental encoder signal through the use of a voltage-to-frequency convertor circuit, such that the frequency of the xe2x80x9cA quad Bxe2x80x9d signal varies in accordance with the rate of change of the measurement. Moreover, in this embodiment, a feedback loop is utilized to ensure accurate representation of the sensor measurement by the incremental encoder signal.
Sensors can provide output signals in a variety of formats. For example, a linear position transducer can provide a pulsed signal comprising a pair of pulses which represent the measured position by the time between the pulses. Such a transducer can also provide an analog voltage signal having an amplitude which represents the measured position, or a digital signal having a binary value which represents the measured position. These signals are all considered xe2x80x9cabsolutexe2x80x9d measurement signals because they have one characteristic which represents the absolute value of the measured variable at all times. Accordingly, with an absolute measurement signal, the value of the measured variable can be determined by simply examining the measurement signal at any instance in time; previous characteristics of the signal are not relevant.
However, many industrial controllers and motion control devices require the measured variable to be represented in an xe2x80x9cincrementalxe2x80x9d format, rather than in an absolute format. An optical encoder is an example of a device which indicates a change in a variable through the use of an incremental signal, such as a series of pulses, each pulse indicating an incremental change in the variable. The controller which receives this incremental output counts the pulse transitions to determine the variable measured by the encoder. Thus, the controller cannot evaluate an incremental signal at any instance in time to determine the value of the variable; the previous pulses in the incremental signal are relevant to determining the relevant to determining the value of the variable.
One typical incremental signal provided by an encoder is referred to as an xe2x80x9cA quad Bxe2x80x9d signal, in which two trains of pulses (A and B) are provided which are 90 degrees out of phase from one another (xe2x80x9cin quadraturexe2x80x9d). As shown in FIGS. 1a and 1b, signal A is typically a square wave alternating between the logical level 0 and the logical level 1, while signal B mimics A but either lags or leads A by 90 degrees, depending upon the direction of movement of the variable being measured. For example, the phase relationship of B leading A, shown in FIG. 1a, could represent an increase in the measured variable, and the phase relationship of A leading B, shown in FIG. 1b, could represent a decrease in the measured variable. Each transition of the A or B square wave from high to low or low to high represents one incremental movement in the measured variable. The A and B output signals are delivered to a signal processing circuit, which may include a bidirectional counter and/or associated software to keep a running count of the output pulses. The count is increased when the measured variable moves in the positive direction, as indicated by the phase relationship of the A and B signals, and the count is decreased when the measured variable moves in a negative direction. After determining the count (which represents the measured variable), the controller can then regulate the machine or process, as known in the art.
Because many controllers require an incremental input to accurately control the machine or process, it is desirable to convert an output signal from an absolute format, such as a digital, analog, or pulsed signal, to an incremental format, such as an xe2x80x9cA quad Bxe2x80x9d square wave format. Making such a conversion allows a controller requiring incremental signals for input to be used with a sensor having an absolute measurement output.
U.S. Pat. No. 5,206,586 discloses a linear position magnetostrictive transducer which allegedly generates an emulated square-wave-in-quadrature encoder output, providing quadrature pulses indicating the location of a position magnet along the length of a magnetostrictive gauge. The patent discloses a microprocessor-based conversion system in which a raw count, which is proportional to the physical location of the position magnet, is converted into quadrature pulses, the number and frequency of which purportedly vary with magnet position and velocity. However, such a microprocessor-based system is limited by the microprocessor clock frequency. Accordingly, if the microprocessor clock frequency is slower than the speed at which the magnet is moved, velocity will not be accurately represented by the synthesized encoder signal. In particular, the position represented by the encoder signal will significantly lag the actual position of the magnet during this high velocity movement.
Thus, there remains a need for an improved method and apparatus to convert an absolute sensor output signal to an incremental signal, such as an xe2x80x9cA quad Bxe2x80x9d encoder signal for example, which can represent both position and velocity in a highly accurate manner and with high resolution. In addition, there remains a need for a method and apparatus to convert an absolute sensor output signal to an incremental signal which is not limited by a fixed clock for conducting the conversion.
One object of the present invention is to obviate the above-described problems.
It is another object of the present invention to provide a method and apparatus for converting an absolute measurement signal to an incremental signal with high accuracy and high resolution.
A further object of the invention is to provide a method and apparatus for synthesizing an incremental signal that is not limited by a fixed-clock frequency and/or by a microprocessor.
Yet another object of the invention is to convert an absolute signal representing a measured variable to a series of pulses with minimal lag time between the absolute signal and the pulse count.
Another object of the present invention is to provide a method and apparatus to convert an output signal to an incremental signal while automatically correcting for discrepancies between the two signals.
Another object of the invention is to provide an absolute signal to incremental signal conversion method and apparatus which can automatically compensate for errors which occur during operation due to component limitations.
One object of the present invention is to provide a synthesized incremental output signal generator which is driven by a variable frequency signal.
It is another object of the present invention to provide a method and apparatus for converting a linear position transducer output signal to an incremental encoder signal with high accuracy and resolution in velocity and position.
To achieve the foregoing and other objectives, a method of producing an incremental signal from an absolute measurement signal is provided. The method comprises receiving a rate of change of an absolute measurement signal, and generating a variable frequency signal based upon the rate of change. The frequency of the variable frequency signal is proportional to the rate of change. The method also comprises providing a first incremental signal comprising a series of pulses. The frequency of the pulses is based upon the frequency of the variable frequency signal.
According to another aspect of the invention, a system for using an absolute measurement signal to generate an incremental signal is provided. The system comprises a sensor configured to provide an absolute measurement signal, and a calculation circuit in communication with the sensor and configured to determine the rate of change of the absolute measurement signal. The system also comprises a variable frequency generation circuit and an incremental signal generator circuit in communication with the variable frequency generator circuit. The variable frequency generator circuit is configured to provide a variable frequency signal whose frequency varies in proportion to the rate of change. The incremental signal generator circuit is configured to provide a first series of pulses whose frequency is proportional to the frequency of the variable frequency signal, and to provide a second series of pulses whose phase is shifted with respect to the first series of pulses.
A method for producing an incremental signal from an absolute measurement signal is also provided. The method comprises providing an absolute measurement signal, determining the difference between the absolute measurement signal and a synthesized measurement signal, and generating a first incremental signal and a second incremental signal based upon the difference. The first and second incremental signals are out of phase. The method also comprises decoding the incremental signals to generate the synthesized measurement signal.
Still other objects of the present invention will become apparent to those skilled in this art from the following description wherein there is shown and described preferred embodiments of this invention, including a best mode currently contemplated for carrying out the invention, simply for the purposes of illustration. As will be realized, the invention is capable of other different aspects and embodiments without departing from the scope of the invention. Accordingly, the objects, drawings, and descriptions are illustrative in nature and not restrictive in nature.